1. Field of the Invention
The present invention relates generally to an organic electroluminescent device, and more particularly, to an organic electroluminescent device characterized in that light having a high luminance can be stably emitted, and light having a predetermined color can be emitted.
2. Description of the Related Art
In recent years, the needs of flat panel display devices, the consumption of electric power and the size of which are smaller than those of a CRT (Cathode Ray Tube) which has been heretofore generally employed, have been increased as information equipments are diversified, for example. An electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided depending on a used material, that is, roughly divided into an inorganic electroluminescent device using an inorganic material and an organic electroluminescent device using an organic material.
The organic electroluminescent device is so adapted that holes injected from a hole injection electrode and electrons injected from an electron injection electrode are recombined with each other in the interface between an emitting layer and a carrier transport layer and in the emitting layer, and has the advantage that it can be driven at a lower voltage, as compared with the inorganic electroluminescent device.
In the case of the organic electroluminescent device, a light emitting device emitting light in a suitable color can be obtained by selecting a luminescent material. It is expected that the organic electroluminescent device can be also utilized as a multi-color or full-color display device, for example.
In recent years, various studies have been conducted on such an organic electroluminescent device.
In such an organic electroluminescent device , an emitting layer and a carrier transport layer which is constituted by a hole transport layer for transporting holes to the emitting layer and an electron transport layer for transporting electrons thereto are generally provided between the hole injection electrode and the electron injection electrode.
Generally used as the structure of the organic electroluminescent device are a three-layer structure referred to as a DH structure obtained by laminating a hole transport layer, an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode, a two-layer structure referred to as an SH-A structure obtained by laminating a hole transport layer and an emitting layer between a hole injection electrode and an electron injection electrode, and a two-layer structure referred to as an SH-B structure obtained by laminating an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode.
An electrode material having a large work function, for example, gold or an indium-tin oxide is used as the hole injection electrode, and an electrode material having a small work function, for example, Mg is used as the electron injection electrode.
An organic material having the property of a p-type semiconductor is used as a hole transporting material in the hole transport layer, and an organic material having the property of an n-type semiconductor is used as an electron transporting material composing the electron transport layer. The property of a material composing the emitting layer is determined depending on the device structure of the organic electroluminescent device. For example, the material having the property of the n-type semiconductor, the material having the property of the p-type semiconductor, and the material having the property close to neutrality are respectively used in the SH-A structure, the SH-B structure, and the DH structure.
Conventionally, a chelate metal complex such as tris(8-quinolinolato)aluminum (hereinafter referred to as Alq3) indicated by the following structural formula 1 has been generally used as the electron transporting material composing the electron transport layer. 
The above-mentioned chelate metal complex is superior in film stability, but does not have sufficient electron transporting properties. In order to transport electrons in sufficient quantity to emit light having a high luminance, a driving voltage to be applied to the organic electroluminescent device must be increased.
Conventionally used as the hole transporting material composing the hole transport layer is a material having a phenylamine structure such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-1,1xe2x80x2-diphenyl-4,4xe2x80x2-diamine (hereinafter abbreviated as TPD) indicated by the following structural formula (2). 
Although the above-mentioned material having a phenylamine structure is superior in hole transporting properties to some extent, the hole transporting properties are not necessarily sufficient.
Although the electron transport layer has the properties of transporting electrons, and the hole transport layer has the properties of transporting holes, they are very poor in the properties of transporting carriers.
Therefore, holes and electrons are respectively stored in the interface between the electron transport layer and the emitting layer and the interface between the hole transport layer and the emitting layer. Consequently, the luminescent properties, the life, and the like of the organic electroluminescent device are reduced.
The material composing the emitting layer is generally low in the properties of transporting electrons and holes. Therefore, electrons and holes are not sufficiently injected into the emitting layer, so that sufficient luminance cannot be obtained.
Conventionally used as the organic electroluminescent device is one obtained by doping a dopant having high luminescent properties into a host material in an emitting layer to obtain sufficient luminance.
Excitation energy is not satisfactorily moved to the dopant from the host material depending on the types of the host material and the dopant which are used for the emitting layer. Therefore, sufficient luminance may not, in some cases, be obtained.
Furthermore, the host material also emits light in addition to the dopant depending on the types of the host material and the dopant which are used for the emitting layer, so that light having a suitable color cannot be emitted. Particularly, it is very difficult to emit red light having a high luminance and having a high color purity.
An object of the present invention is to increase hole transporting properties in a hole transport layer and electron transporting properties in an electron transport layer while maintaining film stability in the hole transport layer and the electron transport layer, to make it possible to stably emit light having a high luminance.
Another object of the present invention is to make it easy to inject electrons and holes into an emitting layer, to make it possible to emit light having a high luminance.
Still another object of the present invention is to satisfactorily move excitation energy from a host material in an emitting layer to a dopant having luminescent properties, to emit light having a high luminance.
A further object of the present invention is to prevent a host material from emitting light in addition to a dopant in an emitting layer, to emit light in a suitable color.
In an organic electroluminescent device having a carrier transport layer and an emitting layer which are composed of an organic material provided between a hole injection electrode and an electron injection electrode, a first organic electroluminescent device according to the present invention is characterized in that a dopant for transporting carriers is doped into the carrier transport layer.
When the carrier transport layer is an electron transport layer, it is possible to dope as the dopant for transporting carriers a dopant having electron transporting properties, a dopant having hole transporting properties, and a dopant having both electron transporting properties and hole transporting properties.
When the dopant having electron transporting properties is doped into the electron transport layer, an electron transporting material superior in film stability, for example, the above-mentioned Alq3 is used as a host material in the electron transport layer, while a dopant having high electron transporting properties is doped into the host material. Consequently, an electron transport layer high in film stability and superior in electron transporting properties is obtained, and electrons in sufficient quantity are transported through the electron transport layer. Accordingly, light having a high luminance is stably emitted. In doping the dopant having high electron transporting properties as described above, the electron transporting properties in the electron transport layer cannot be considerably improved if the amount thereof is small, while stability in the electron transport layer is decreased if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the electron transport layer is in the range of 0.01 to 50% by weight.
When the dopant having hole transporting properties is doped into the electron transport layer, holes introduced into the electron transport layer from the emitting layer by the dopant having hole transporting properties are introduced into the electron transport layer. Accordingly, holes are prevented from being stored in the interface between the electron transport layer and the emitting layer. Therefore, the luminescent properties, the life, and the like in the organic electroluminescent device are improved. In thus doping the dopant having hole transporting properties, holes cannot be sufficiently prevented from being stored in the interface between the electron transport layer and the emitting layer if the amount thereof is small, while the electron transporting properties in the electron transport layer are degraded if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the electron transport layer is in the range of 0.01 to 50% by weight.
When the carrier transport layer is a hole transport layer, it is possible to dope as the dopant for transporting carriers a dopant having hole transporting properties, a dopant having electron transporting properties, and a dopant having both electron transporting properties and hole transporting properties.
When the dopant having hole transporting properties is doped into the hole transport layer, a hole transporting material superior in film stability is used as a host material in the hole transport layer, while a dopant having high hole transporting properties is doped into the host material. Consequently, a hole transport layer high in film stability and superior in hole transporting properties is obtained, and holes in sufficient quantity are transported through the hole transport layer. Accordingly, light having a high luminance can be stably emitted. In thus doping the dopant having high electron transporting properties, the hole transporting properties in the hole transport layer cannot be considerably improved if the amount thereof is small, while stability in the hole transport layer is decreased if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the hole transport layer is in the range of 0.01 to 50% by weight.
When the dopant having electron transporting properties is doped into the hole transport layer, electrons introduced into the hole transport layer from the emitting layer by the dopant having electron transporting properties are introduced into the hole transport layer. Accordingly, electrons are prevented from being stored in the interface between the hole transport layer and the emitting layer. Therefore, the luminescent properties, the life, and the like in the organic electroluminescent device are improved. In thus doping the dopant having electron transporting properties, electrons cannot be sufficiently prevented from being stored in the interface between the hole transport layer and the emitting layer if the amount thereof is small, while the hole transporting properties in the hole transport layer are degraded if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the hole transport layer is in the range of 0.01 to 50% by weight.
When the dopant having two properties, that is, electron transporting properties and hole transporting properties is doped as the dopant for transporting carriers into the electron transport layer and the hole transport layer, electrons and holes are transported by the dopant. Therefore, the holes are prevented from being stored in the interface between the electron transport layer and the emitting layer, and the electrons are prevented from being stored in the interface between the hole transport layer and the emitting layer. Therefore, the luminescent properties, the life, and the like in the organic electroluminescent device are improved.
In an organic electroluminescent device having a carrier transport layer and an emitting layer which are composed of an organic material provided between a hole injection electrode and an electron injection electrode, a second organic electroluminescent device according to the present invention is characterized in that a dopant for transporting carriers or moving excitation energy is doped into the emitting layer.
As in the second organic electroluminescent device, when the dopant for transporting carriers or moving excitation energy is doped into the emitting layer, electrons and holes are easy to inject into the emitting layer by the dopant. Therefore, the probability that the electrons and the holes are combined with each other in the emitting layer to emit light is increased. Further, a luminescent material in the emitting layer is efficiently excited, so that light having a high luminance is emitted at a low voltage.
When the dopant for transporting carriers or moving excitation energy does not emit light, only the luminescent material in the emitting layer emits light. Therefore, light in a predetermined color is emitted.
Used as the dopant for transporting carriers or moving excitation energy is one whose energy gap is larger than the energy gap of the luminescent material in the emitting layer. Examples are rubrene, 9,10-diphenyl anthracene, and anthracene which are condensed polycyclic aromatic compounds.
In doping the dopant for transporting carriers or moving excitation energy into the emitting layer, the probability that the electrons and the holes are combined with each other to emit light in the emitting layer is increased, and the luminescent material in the emitting layer cannot be efficiently excited if the amount thereof is small, while the amount of the luminescent material in the emitting layer is decreased, so that light cannot be sufficiently emitted if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the emitting layer is in the range of 0.01 to 50% by weight.
In an organic electroluminescent device having a carrier transport layer and an emitting layer which are composed of an organic material provided between a hole injection electrode and an electron injection electrode, a third organic electroluminescent device according to the present invention is characterized in that a dopant having luminescent properties and a dopant for moving excitation energy which assists the dopant having luminescent properties to emit light are doped into the emitting layer.
As in the third organic electroluminescent device, when the dopant having luminescent properties and the dopant for moving excitation energy which assists the dopant having luminescent properties to emit light are doped into the emitting layer, the dopant having luminescent properties is efficiently excited by the dopant for moving excitation energy. Therefore, light having a high luminance can be emitted at a low voltage.
When the dopant for moving excitation energy does not emit light, only the dopant having luminescent properties which is excited by the dopant for moving excitation energy emits light. Therefore, light having a high color purity is obtained.
In doping the dopant having luminescent properties or the dopant for moving excitation energy which assists the dopant having luminescent properties to emit light into the emitting layer, the dopant having luminescent properties which is doped into the emitting layer cannot be efficiently excited, so that sufficient luminance cannot be obtained if the amount thereof is small, while a problem arises in stability or the like in the emitting layer if the amount thereof is too large. Therefore, it is preferable that the amount of the dopant in the emitting layer is in the range of 0.01 to 50% by weight.
In order to emit red light having a high luminance and having a high color purity in the third organic electroluminescent device, a compound, selected from the following structural formulas (3) to (5), having a luminescent peak wavelength in the range of 550 nm to 700 nm, for example, is used as the dopant having luminescent properties, and rubrene is used as the dopant for moving excitation energy. 
R1 to R7 in the foregoing structural formulas (3) to (5) are hydrogen or a substituent. Examples of R1 to R7 include xe2x80x94CnH2n+1, xe2x80x94CN, xe2x80x94O(CnH2+1), xe2x80x94N(CnH2n+1), a hydrogen group, a phenyl group, and a naphthyl group, where n is an integer of 0 to 10.
The device structure of each of the first to third organic electroluminescent devices in the present invention may be any one of known structures, that is, a DH structure obtained by laminating a hole transport layer, an emitting layer, and an electron transport layer between a hole injection electrode and an electron injection electrode, an SH-A structure obtained by laminating a hole transport layer and an emitting layer between a hole injection electrode and an electron injection electrode, and an SH-B structure obtained by laminating an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.